Ink jet printing apparatus having one or more ink jet heads for projecting drops of ink onto paper or other printing medium to generate graphic images and text have become increasingly popular. To form color images, apparatus with multiple ink jet printing heads are used, with each head being supplied with ink of a different color. These colored inks are then applied, either alone or in combination, to make a finished color print. Typically, all of the colors needed to make the print are produced from combinations of cyan (a blue-green), magenta (a blue-red), a yellow ink. In addition, black ink may be utilized for printing textual material or for producing true four color prints.
In a common arrangement, the print medium is attached to a rotating drum, with the ink jet heads being mounted on a traveling carriage that traverses the drum axially. As the heads scan spiral paths over the medium, ink drops are projected from a minute orifice in each head to form an image on the medium. A suitable control system synchronizes the generation of ink drops with the rotating drum.
There are two basic types of ink jet printing systems. In the first type, a stream of ink drops is produced continuously, but the stream is deflected away from the medium except when printing is desired. In the other type, ink drops are produced on demand. One such drop-on-demand printer is illustrated in U.S. Pat. No. 4,106,032 of Miura et al. In the Miura printer, an electric pulse applied to a piezoelectric crystal causes it to constrict whenever a drop of ink is needed. As a result, because the crystal is in intimate mechanical contact with an ink chamber in the print head, a pressure wave is transmitted through the ink chamber. This causes the formation at an internal drop-forming orifice of an ink drop, which is projected toward the printing medium. As they move toward the main external orifice leading to the medium, the drops of ink are entrained in a concentric air stream. This air stream increases the speed of the drops and accuracy of applying the drops to the print medium.
These known devices suffer from a number of drawbacks. Particulate material and air bubbles in the ink supplied to the print heads quickly clog the internal drop-forming orifice. Furthermore, pressure transients are generated in the ink supplied to the ink jet heads. These transients result from external factors, such as vibrations induced when ink cartridges are replaced, when the apparatus is jarred during use, and when the apparatus is moved. Whenever these ink pressure transients occur, and pressure in the ink supply lines drops, air may be taken in or ingested into the ink drop-forming orifice of the print head. This ingested air forms a bubble which clogs the print head and causes it to malfunction. Furthermore, it is a relatively time consuming and messy task to change the print heads of these prior art devices when a malfunction occurs.
U.S. Pat. No. 4,347,524 of Engel discloses a device which has a shock absorber for suppression of high frequency ink pressure transients in an ink jet printer. In a first embodiment, Engle feeds ink through a constricted piece of tubing and then through a chamber, partly filled with air, to an ink jet head. The air bubble in the chamber, together with the resistance created by the constricted tubing, forms a shock absorbing mechanism that is analagous to an electrical resistor-capacitor (RC) low pass filter. The capacitance of this device is dependent on the amount of air in the chamber. Furthermore, ink passing through the chamber can add to or absorb air from this bubble. As ink absorbs the air, the capacitance decreases. As a result, the RC time constant of the system decreases and degrades the performance of the system in damping pressure transients. In addition, during movement of the device air bubbles can be formed in the ink, leading to possible failure.
In his second embodiment, Engel positions a flexible diaphragm wall in his chamber. Although this eliminates problems with an air and ink interface, other drawbacks exist. For example, one side of the Engel diaphragm is apparently exposed to the atmosphere while the other side is exposed to ink. As a result, a relatively high pressure differential develops across the diaphragm. This decreases the capability of the Engel apparatus to act as a fluid capacitor. As a result, the RC time constant of the system decreases.
Moreover, none of the Engel embodiments prevents air bubbles and particles in the ink supply stream from reaching and clogging the ink jet print head. In addition, the Engel apparatus does not effectively dampen low frequency, long duration pressure drops in the ink supply line. Such pressure transients also may cause the ingestion of an air bubble into the ink jet head and a corresponding clogging of the print head.
IBM Technical Disclosure Bulletin, Volume 25, No. 2, pages 772-774 published in July 1982 contains two bulletins directed to ink jet printer devices. One of the bulletins discloses the use of check valves in an ink jet printer system for the elimination of reverse ink flow. The purpose of these check valves is to prevent paper fibers and other contaminants from being drawn into the print head where they could plug the head. Thus bulletin also shows a filter positioned within an ink inlet line of an ink jet head for filtering contaminants from ink that is diverted away from the printing medium. The other bulletin discloses the use of a surge pressure orificed check valve in an ink jet printer. The IBM devices do not, however, satisfactorily address the problem of air ingestion into an ink jet head. Such air ingestion results from, for example, relatively high frequency pressure transients that can occur in an ink supply line.
Therefore, a need exists for an improved ink jet printing apparatus directed to overcoming these and other disadvantages of prior art devices.